In an automobile engine, in order to achieve the dual objectives of improved torque at low or medium running speeds and improved output at high running speeds, a plurality of cams are used selectively according to the engine running conditions. By using cams of different shapes, the lift properties of an intake valve and an exhaust valve are varied. In this way, the timing with which air is taken into and exhaust gas is expelled from the engine, and the air intake and exhaust gas volume, are controlled.
A cam change-over mechanism for such an engine is described for example in Japanese Tokkai Sho 63-167016 and Tokkai Sho 63-57805 published by the Japanese Patent Office.
In this change-over mechanism, a low speed rocker arm and high speed rocker arm are supported adjacent to each other on a common rocker shaft such that they are free to oscillate independently of each other, the end of the low speed rocker arm being in contact with the intake valve or exhaust valve. A low speed cam slides on the low speed rocker arm, and a high speed cam slides on the high speed rocker arm. The high speed rocker arm has a shape corresponding to a longer valve opening time or larger valve lift than the low speed cam.
Engaging holes parallel to the rocker shaft are formed in both rocker arms at a predetermined distance from the shaft, a plunger being inserted in one of these holes.
This plunger is driven by oil pressure. When the low speed and high speed rocker arms come to a predetermined rotational position relative to one another, the plunger is pushed out into ,.the engaging hole in the adjacent rocker arm so that the two rocker arms lock together. The valve then opens and closes according to the characteristics of the high speed cam. When the plunger withdraws from the hole in the rocker arm, the two rocker arms are released, and the valve opens and closes according to the characteristics of the low speed cam.
In this cam change-over mechanism, however, the engaging holes in the rocker arms must be arranged coaxially when the plunger moves. The plunger and the engaging holes therefore have to be machined with extremely high precision so that locking and release of the rocker arms is smooth. As a result, the machining of these parts is difficult and costly.
Further, to maintain the anti-wear properties of the engaging parts, an iron alloy is generally used to manufacture them. This however increases the inertial mass of the valve drive mechanism, and impairs the response of the valve in response to the action of the cams in the high engine speed region.